Mediated electron transfer development

Redox processes are fairly common in the formation of Z —CO— complexes of transition metals, and an example is given in Eq. (9). In this reaction, titanium is oxidized from the + 2 to the +3 state, thus becoming a better Lewis acid, and the molybdenum dimer is reductively cleaved, thus developing Z —CO— donor character (59). A characteristic low-frequency Z —CO— band is observed in the IR spectrum, and a crystal structure is available. A proposed mechanism for the redox process, based on CO mediated electron transfer, is discussed in Section IV,C. 

Redox hydrogels containing coordinated metal complexes that have a formal potential which enables mediated electron transfer to and from interfacial electroactive biomolecules are highly attractive for biosensor development [11]. For example, redox hydrogels are the only known... 

In order to screen mutants with improved direct electron transfer, it is necessary to use an electrochemical screening system. Currently, only a few electrochemical screening methods were described in literature such as the system developed by the Bartlett group used to screen NADH electro-oxidation. This system uses a multichannel potentiostat with sixty electrodes to screen zinc(n) or ruthenium(ii) complexes bearing the redox phenidione as a mediator for NADH oxidation. It allows the complete evaluation of the electrochemical kinetic constants of the mediators and the immobilization procedure. Unfortunately, this system could only be used with a single electrolyte solution for all the electrodes (e.g., when a single reaction condition or enzyme is assayed), and it requires mL-scale reaction volumes. Recently, another system was described which makes it possible to screen bioelectrocatalytic reactions on 96 independent electrodes screen-printed onto a printed-circuit-board. It showed the possibility to screen direct or mediated electron transfer between oxidoreductases and electrode by intermittent pulse amperometry at the pL-scale (Fig. 6). The direct electron transfer assay was validated with laccase and unmodified electrodes.As an example of the mediated electron transfer assay, the 96 carbon electrodes were modified by phenazines to sereen libraries of a formate dehydrogenase obtained by directed evolution. ... 

To explore the utility of DNA-mediated electron-transfer events for biosensing applications, an electrochemical assay was developed [23-25]. Electrochemical sensors are typically more portable and inexpensive than those using fluorescence-based detection, hence a system suitable for monitoring electron transfer through DNA duplexes immobilized on the surface of an electrode was sought. 

When discussing the transfer of electrons from the enzyme active site to the electrode surface, thus generating catalytic current, there are two types of electron transfer mechanisms mediated electron transfer (MET) and direct electron transfer (DET) [13]. Most oxidoieductase enzymes that have been commonly used in BFC development are unable to promote the transfer of electrons themselves because of the long electron transfer distance between the enzyme active site and the electrode surface as a result, DET is slow. In such a case, a redox-active compound is incorporated to allow for MET. In this approach, a small molecule or redox-active polymer participates directly in the catalytic reaction by reacting with the enzyme or its cofactor to become oxidized or reduced and diffuses to the electrode surface, where rapid electron transfer takes place [14]. Frequently, this redox molecule is a diffusible coenzyme or cofactor for the enzyme. Characteristic requirements for mediator species include stability and selectivity of both the oxidized and reduced forms of the species. The redox chemistry for the chosen mediator is to be reversible and with minimal overpotential [15]. 

On the other hand, if the hole flow in DNA could be artificially controlled to deposit at the desired site in DNA, it may enable site-selective oxidation and strand scission of DNA, which is desirable from a therapeutical standpoint. Furthermore, understanding DNA-mediated hole transfer is expected to lead to an additional application in the development of biosensors and bioelectronic devices [9]. Therefore, the regulation of the transfer rate and direction of the hole generated in DNA is of interest from the perspective of using DNA as a building block for electronic devices. 

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